The present invention relates to a beam hardening (sometimes abbreviated as ‘BH’ hereinbelow) post-processing method and an X-ray CT apparatus for correcting intensity of X-rays passing through a subject based on phantom data.
An X-ray source employed in an X-ray CT apparatus outputs X-rays having a certain range of energy. On the other hand, the linear absorption coefficient for X-rays passing through a subject depends upon the X-ray energy, and exhibits a beam hardening effect by which the average energy shifts to a higher level as the transmission length through the subject becomes larger. Thus, the X-ray transmission intensity, i.e., projection information value, and the transmission length do not maintain a proportional relationship with each other, i.e., are non-linearly related to each other.
The BH effect results in a cupping effect by which intensity decreases in the central portion of a reconstructed image, and is corrected by, for example, determining a corrective function for each channel of an X-ray detector for correcting the projection information values to produce a reconstructed image with homogeneous intensity (see Patent Document 1, for example).
Moreover, to achieve correction with higher accuracy, a plurality of cylindrical phantoms of different diameters, which diameters are sufficient to generally cover the entire FOV (i.e., imaged region), disposed at an imaging center are imaged, and projection information of the phantoms are used to improve the accuracy of the aforementioned correction.